crimson/mon: fallback to msgr v2 for unbound msgr

crimson/mon: fallback to msgr v2 for unbound msgr

so, for instance, if we want to connect to monitor without bind to any
address, we can try to use the v2 addresses advertised in monmap or
local settings, instead of being unable to connect to mon because we are
using an `entity_addr_t::TYPE_NONE` address which is returned by
`entity_addrvec_t::front()` if the addrvec is empty.

see also AsyncMessenger::should_use_msgr2().

Signed-off-by: Kefu Chai <kchai@redhat.com>